There has already been proposed (for example, Patent Literature 1 and 2) a speed reducer including a first circular plate on an input side having a ball engagement groove, a second circular plate on an output side having a ball engagement groove, a plurality of balls which engage with the ball engagement grooves, and a retainer interposed between the first circular plate and the second circular plate, and configured to hold the balls. The speed reducer of this type is excellent in that, for example, a high speed reduction ratio and a small size is achieved.
The speed reducer proposed in Patent Literature 1 includes first and second circular plates each having a meandering first groove crossing a first reference circle on a plane at a constant pitch and a meandering second groove crossing a second reference circle on a plane at a constant pitch. The first circular plate and the second circular plate are opposed to each other through intermediation of first and second rolling elements respectively held by first and second retainers. The first retainer is fixed, and the second retainer is supported so as to be rotatable. It is described that this speed reducer is of a differential type. Thus, a high speed reduction ratio and a small size can be obtained.
A speed reducer proposed in Patent Literature 2 includes a drive cam, a driven cam, and a retainer. The drive cam has a circular cam groove which engages with balls, and is eccentric by a certain distance with respect to a rotary shaft. The driven cam has a cam groove having a shape of petals which engages with the balls. The retainer has grooves configured to hold the balls so that the balls are movable in a radial direction. The drive cam and the driven cam are arranged on both sides of the retainer while surfaces thereof having the cam grooves are opposed to each other, and are coupled to each other so as to be rotatable about the same axis. Rotation of the drive cam is reduced in speed through actions of the balls, and is transmitted to the driven cam. It is described that this speed reducer is small in size and is manufactured at low cost by relatively simple machining, and that a speed reduction ratio of approximately 6 can be obtained.